Alberto Prieto / AP Photo

The resignation of Kofi Annan, the U.N. and Arab League special envoy for Syria, made official what was long ago apparent. His six-point peace plan was going nowhere because the Syrian regime and opposition weren’t following it, and key countries including Russia, China, and the United States weren’t able to agree on an acceptable solution to the crisis, much less impose it on the combatants.

It may still be true that President Bashar al-Assad’s days are numbered, but his last day is some time off. For the moment, diplomatic options have been spent. The civil war is on and it will be a fight to an uncertain finish. The conflict could outlast Assad, however and whenever he falls.

Government forces, despite steady defections, remain loyal to Assad. They retain considerable firepower and are increasingly willing to use all that they have–tanks, helicopters, and fixed-wing aircraft–to put down the popular revolt. Whether or not Assad is prepared to use chemical weapons against the rebels, they have undoubtedly deterred an overt outside intervention. The lesson for other rogues like Iran and North Korea could not be clearer.

But over 18 months, the Syrian regime has been unable to subdue an emboldened opposition that against steep odds is not only holding its own, but territory as well. The opposition’s capability is increasing, due to support both inside and outside Syria. Two weeks ago, it was able to penetrate Assad’s inner circle, killing three senior government officials. At least thus far, it has fought the government to a draw in the battle raging in Aleppo.

Given the formal end of the Annan peace mission, questions have already been raised about outside military options, including intervention. In fact, an intervention is already underway.

A number of countries are already deeply involved in Syria–it is at one level a proxy battle between the Gulf States and Iran. The United States has been providing non-lethal assistance, and there are reports President Obama has authorized covert assets to assess what is happening, sort out who the players are, provide appropriate support, and make their efforts more effective.

But in contrast to Libya, there will be no large-scale military intervention. The focus will be on containing the crisis within Syria, helping the fractured Syrian opposition pull together, and developing a plan to prepare for the day after the conflict ends.

There are lots of reasons for this, and some important implications as well.

Libya represented an ideal confluence of events–an unprecedented Arab League call for intervention, a U.N. Security Council authorizing resolution, and an alliance with key members willing to undertake the mission. None of those conditions exist in the context of Syria.

The Arab League is divided. Multiple Russian and Chinese vetoes have locked up the UN. Two key NATO leaders from a year ago – Nicholas Sarkozy and Silvio Berlusconi – have been pushed out of office. For other European leaders, the survival of the Eurozone is now a preoccupation. For Barack Obama, there is a reelection campaign.

And notwithstanding the emerging tragedy in Syria, the graver regional threat remains Iran and its nuclear ambitions.

Russia and China have ensured that Libya serves as a future exception rather than a rule. They have effectively killed the emerging concept of “responsibility to protect,” which they viewed in Libya (with some justification) as a pretext for regime change. Russian and Chinese intransigence carries diplomatic costs–finger-pointing and name-calling are actually appropriate in this case–which both countries, for their own domestic reasons, are prepared to pay.

Conservatives in this country will undoubtedly pile on the United Nations for its ineffectiveness. But the fact that it has fallen short of its ideal is less about the institution itself than the will of its key members. As always, the UN is only as effective as the Security Council wants it to be. Kofi Annan’s resignation exposes that inconvenient truth.

Obama’s political opponents will try to portray his caution regarding Syria as evidence of the decline of American leadership. And yet, despite the horrific pictures emerging from Aleppo–there has been no CNN, al Jazeera, or YouTube Effects regarding Syria–he faces little political pressure to militarily intervene. Many want him to “do something,” but most proposals deal with the effects of the crisis, not the conflict itself.

Obama is rightly skeptical about military interventions, which have traditionally been couched in lofty rhetoric and fallen short in actual results. The Libyan intervention–regime change for just over one billion in expenditure–is the model of a new emerging strategy that stresses clear strategic ends, employs fewer boots on the ground, and carefully weighs risks, costs, and benefits.

But the Libya intervention cuts both ways. Estimates of casualties in Syria are almost certain to surpass those in Libya. If the Libyan people have a right to protection from a ruthless dictator, why don’t the Syrian people? Obama decisively called for Muammar Qaddafi to step aside, but hesitated for months to say the same thing about Assad. The administration has struggled to balance short-term interests and long-held values and construct a strategy and narrative where actions consistently match words.

Kofi Annan’s resignation ensures that come January, President Obama or President Romney will inherit a violent, dangerous, and complex crisis in Syria. Over the final three months of the campaign, the two candidates should be challenged to articulate what to do about Syria: what is at stake; how to square our interests and values; what military, economic, and diplomatic tools can make a difference; and what actions can be taken alone and with others to save Syria from what Annan termed “the worst calamity.”